1. Field of the Invention
The present invention relates generally to rechargeable battery systems for electric vehicles, and more particularly, to protection for such a battery.
2. Description of the Prior Art
Electric vehicles commonly use a rechargeable battery to provide energy for operating the vehicle's motor. The battery is discharged as it is used. Discharging a rechargeable battery below a certain critical voltage, commonly known as a cut-off voltage, may result in permanent battery damage.
It is desirous to protect the battery from damage by sensing when the battery voltage is at the cut-off, and take steps such as disconnecting the battery from loads such as the electric motor in the vehicle, to prevent battery damage from occurring. It is equally desirous to allow the user or operator to use the battery fully. As should be apparent, these two objectives can be in conflict.
To protect the battery from damage, yet allow the battery to be used to its capacity, the battery must be monitored and action taken when the battery reaches its cut-off voltage.
A problem arises, however, in comparing the measured battery voltage to the cut-off voltage. The cut-off voltage specified by a battery manufacturer is a measurement made in a no load condition, when the battery is not operating devices such as motors, lights, or other electrical devices.
As explained further elsewhere herein, the measured voltage of a battery under load, when the battery is operating, for example, an electric vehicle with an electric motor, lights, and other equipment, is lower than the measured voltage of the battery when it is in a no load condition, as is the case when the battery is not supplying power to those devices.
Because the measured voltage of the battery under load is lower, stopping use of the battery when its measured voltage under load is below the cut-off voltage could stop use of the battery prematurely, when there is still useable energy left in the battery, for example, for operating the vehicle.
The measured voltage of a battery under load is lower than the measured voltage of the battery under no load because of the internal resistance of the battery.
This lowered voltage, or voltage drop, caused by the internal resistance of the battery is commonly referred to as sag. As the load on the battery increases, and current drawn from the battery increases, the sag caused by internal resistance increases, and the measured battery voltage decreases. This decrease makes comparison of the measured battery voltage to the cut-off voltage problematic. While such a comparison will protect the battery, it will also stop use of the battery prematurely, with useable charge remaining.
What is needed is a way to protect a battery from being discharged past its cut-off voltage while avoiding the problem of sag induced lower voltage measurements of a battery under load.